Abstract/Summary

The term ‘Chalk rivers’ is used to describe all those water courses dominated by groundwater discharge from Chalk geology. Natural conditions and historical modification have generated an ecosystem, with rich and unique assemblages and with high value to society (e.g. SACs, SSSIs, visual amenity and fisheries. Chalk rivers are considered to be sensitive to hydrological and morphological change and there is concern that flood defence and land drainage schemes, catchment agriculture, urbanisation, climate change and abstraction are leading to a decline in river health. This report reviews tools and methods available for setting ecologically acceptable flows in Chalk rivers to help define sustainable abstraction levels.
Methods of ecological flow setting are classified into four types (1) look-up tables (2) desk (3) functional analysis and (4) physical habitat modelling. The analysis of methods of their application leads to 12 recommendations.
1. Chalk rivers are iconic English rivers. All flow-setting studies should define clear objectives for the river system with costs and benefits of different options.
2. Chalk rivers have some hydrological characteristics in common including the baseflow-dominate response. Low flows, especially in the summer are critical to Chalk river ecosystems, such as August flows for salmon parr.
3. Chalk rivers have ephemeral (winterbourne) reaches which have no flow during certain periods. A research study is needed on ephemeral river flow setting
4. Chalk river ecosystems are sensitive to flow change and setting the appropriate flow is crucial to river conservation. Information on hydromorphology and ecological dynamics needs to be combined.
5. The impact on the flow regime varies according to the type of abstraction. Different management approaches may be needed for these abstraction types.
6. Chalk rivers are characterised by high macrophyte biomass. This needs to be considered in the role of flow in creating habitat.
7. Chalk rivers are not natural. Channel morphology, macrophyte growth, and flow need to be considered together.
8. Chalk rivers are not one homogeneous river type due to management. There is a need for individual studies to define hydraulically appropriate flow regimes.
9. In Chalk rivers the relative sensitivity of habitat types is not clear. Ecological flow studies need to consider the whole reach, not just single habitats (e.g. riffles).
10. In Chalk rivers, flow may be linked to other factors such as temperature. Analysis is required to assess trends and changes in flow and temperature regimes.
11. Flow in Chalk rivers comes primarily from aquifers. Appropriate flow regimes may be achieved at critical times by stream support by pumping from an aquifer.
12. Chalk rivers tend to be low in fine sediment and recover slowly from artificial inputs. Simple channel narrowing is not necessarily a satisfactory solution.
Four areas for new research are proposed.
1. To develop a rapid assessment of physical habitat sensitivity to flow change for Chalk rivers
2. To investigate the response of Chalk river biota to the combined effects of flow, morphology, sediment and water quality.
3. To define any trends, changes and variations in environmental conditions in Chalk rivers
4. To examine hydro-ecological issues of ephemeral reaches of Chalk rivers